I took a picture of my son in an elementary school cafeteria. The room was very well lit and the parameters of the shot were Canon T3, 18mm, F/3.5, 1/125, ISO 100. I was close to him and his head basically filled the frame. He was about 20 feet from the wall and against the wall there were tables with books on them.

When I took the picture, he was totally in focus but the wall, table, and books were also in focus, though not as sharp as he was. I expected to get a very blurred background and was surprised that I could make out everything 20 ft behind him in such detail.

3 Answers
3

The small degree of defocus in the background is due to the focal length used being very short (18mm).

The amount of background blur depends on the size of the entrance pupil, not the f-number. The entrance pupil size is the focal length divided by f number, so in this case it would be about 5mm. This is quite small. A 100mm lens at f/3.5 would have an entrance pupil of size 29mm.

For this reason you will get more out of focus backgrounds at the other end of the kit zoom range at f/5.6, at 55mm the entrance pupil will be twice the size at 10mm.

I am guessing that you have an 18-55mm. In this plot the background blur between 18mm f3.5 and 55mm f5.6 are compared for a head and shoulders portrait. As you see the background blur at 6m (~20ft) is almost twice as much at 55mm f5.6. The entrance pupil mentioned in a previous answer is only valid when the distance between the subject and the background reaches infinity.

You can see this comparison here. In general there are three factors which give more background blur for a given subject framing:

Could you elaborate a bit on the entrance pupil part? Why is this explanation only valid when the distance between subject and background reaches infinity (maybe a link to an explanation)? Does 6m not approach infinity enough?
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Bart ArondsonMar 6 '13 at 23:35

The equation for background blur is: (focal length * magnification factor / aperture) * (background distance - subject distance) / background distance. The first part of this equation is the entrance pupil part. But effects of the second part are also shown in these graphs. So for the shot of the question you have a subject distance of 3ft and a background distance of 20ft+3ft=23ft. Therefore the difference in entrance pupil has to be multiplied with 20/23. So you are right that for this specific shot the entrance pupil is a good estimation, but it was more a statement in general.
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mmumbossMar 7 '13 at 6:26

Actually, I explained it a little bit wrong. In this comparison, the entrance pupil of the 18mm needs to be multiplied by 20/23. However, in order to get the same framing with a 55mm lens, the subject distance increases, and hence, you need to multiply the part with the entrance pupil with 20/29. So you can see that there is more to it than just the entrance pupil.
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mmumbossMar 7 '13 at 6:58

The DOF depends on the ratio between the distance you put the focus on, and the focal length; and also on the numeric aperture.

You used a low aperture; it helps to keep a low DOF. Buy if your point of focus is, say, 5 meters away and your focal length is 18mm, be sure it is enough to keep in focus the whole scene behind 5 m distance, even with so low aperture number.